How to batch import images as brush textures?
There is an addon just for this, Import BrushSet. You can download the addon from here.

After you have it installed, simply click on File > Import > Brush Set.


You have to choose a folder (don't pick any images, just the folder) and the addon will import all the images in that folder and set them up as brush textures. (It would be a good idea to make a separate folder with only the images you want to import)

It's as simple as that.